Array concat returns empty array

Question

I'm adding several items to an array using concat in an event-handler as follows:

var selectedValues = [];
$.each($('#selected-levels').data("kendoListBox").dataSource.data(), function(i, item) {
    selectedValues.concat([ item.id ])
});

return {
    "selected" : selectedValues
};

this always returns {level-selected: Array(0)} even though I have checked that there are some items in the dataSource (by stepping through with debugger)

why don't the items appear in the array?

Answer 1

concat doesn't mutate the array, you need to set the value back to selectedValues

selectedValues = selectedValues.concat([ item.id ])

Or use push

selectedValues.push( item.id )

Answer 2

Ironically, the answer was already hidden in your question, in your question title more exactly.

Actually concat returns the correct new array you want... you just never used the return value !

You have to be aware that the array is not modified in place, but a fresh copy is returned.

So selectedValues.concat([ item.id ]) should be replaced by selectedValues = selectedValues.concat([ item.id ]) if you want to do anything.